Several processes are already known for carrying out a dry transfer.
First of all, transfer techniques are known which make use of a paper or carrier provided with micro-capsules containing a printing liquid. When a mechanical stress is applied to such a carrier, it causes the micro-capsules to burst, the printing liquid thus being released and so effecting the printing or transfer operation.
Mechanical transfer methods are also known which make use of a carrier having the symbols or patterns which are to be transferred adhering thereto. In this case, the transfer is a true physical transfer of the symbol or pattern from its initial carrier to the receiving carrier. The operation is performed by placing the surface of the initial carrier bearing the symbol or pattern in contact with a receiving carrier and then applying mechanical pressure to the other surface of the initial carrier, for example, by rubbing the surface in question with a pen or scriber.
Processes are also known which make use of the properties of photopolymerisation of particular chemical compounds. Use is made of a donor which carries the symbols which are to be transferred and a coating of a photopolymerisable compound, and of an acceptor on to which the transfer is to be made and which carries a coating of another photosensitive compound. Under the action of light, chemical photopolymerisation reactions occur under particular conditions and which may depend upon the wavelengths of the incident light. These reactions lead to a differential adhesion effect. The symbols or parts which are to be transferred then adhere more strongly to the acceptor than to the donor, and are transferred. The parts which are not to be transferred adhere more strongly to the donor than to the acceptor and are retained on the donor and are not transferred.
The processes referred to above render it possible to perform a transfer by the physical passage of a tangible symbol or element from an initial carrier to which they are initially attached, to an acceptor. This element consists of an ink in the case of processes which make use of microcapsules; it consists of the symbol itself, which is to be transferred, in the other cases.
Processes of colour transfer by thermography are described, for example, in British Pat. No. 953,150 and in U.S. Pat. No. 3,451,338. In these processes, use is made of a chromogenic or colour-producing substance, consisting of a compound of the benzo-indolino-spiropyrane series, which is colourless and translucent in the normal state. When this substance comes into contact with a developer, it becomes coloured and opaque. In accordance with the British patent, the developer consists of a salt of an organic acid whilst according to the U.S. patent, the developer consists of a phenolic compound. The reaction between the chromogenic substance and the developer is effected by the application of heat, for example, by means of infrared radiation.
According to these patents, the chromogenic substance and the developer are mixed with a bonding polymer and deposited upon a carrier, for example, a sheet of paper, in the form of a coating. The transfer of a sign or symbol is effected as a result of the imprint of said sign or symbol in a thermographic ink upon the back of a carrier sheet or upon a template sheet placed in contact with the active coating. Upon exposure to infrared radiation, the thermographic ink selectively absorbs heat and this brings about a chemical reaction between the chromogenic substance and the developer in the corresponding areas of the active coating.
According to another form of procedure disclosed in U.S. Pat. No. 3,451,338, the chromogenic substance and the developer form coatings upon the contacting surfaces of two carrier sheets, one of which comprises one or more signs printed in thermographic ink. Upon heating by infrared radiation or by means of a heating panel (FIG. 4 of the said U.S. patent), the phenolic compound melts in those areas which correspond to the imprint of the thermographic ink and a chemical reaction occurs with the chromogenic substance so that the sign or symbol is printed upon one coating or the other. The migration of the one or the other substance of the reaction is referred to in the said U.S. patent, but it is not explained or analysed in any way.
The process disclosed in British Pat. No. 953,150 found very widespread application in the form of a carrier sheet provided with a single coating comprising both reactive materials, the chromogenic substance and the developer, in an intimate admixture, and on which it is consequently possible to write symbols or letters in colour using a heated scriber or heated characters. This material is utilised on a very large scale at present in printers for cash registers and for computers.
By contrast, if the two reactive materials are situated on two separate sheets and the reaction is brought about by infrared radiation or a heating panel, it was found that the application of a model or pattern of the sign to be transferred in thermographic ink, is essential to secure a useful result. Furthermore, the use of infrared radiation involves a dissipation of heat in all cases, which cannot be rectified and causes accidental transfer of symbols and an absence of clarity in the transferred symbols, which is unacceptable.
The object of the present invention is to provide a transfer material comprising a chromogenic or colour-producing substance and a developer each of which is deposited upon a separate carrier and in which the chemical reaction is brought about by an input of extraneous energy which does not involve any dissipation of heat and permits a perfect transfer of the chromogenic substance on to the carrier of the developer and thus to obtain symbols of perfect clarity.